High alloy steel powders and their consolidation into homogeneous tool steel

ABSTRACT

THE HIGH ALLOY STEEL POWDER CONTAINS ONE OR MORE ELEMENTS WITH A STRONG AFFINITY TO OXYGEN, SUCH AS TITANIUM, CHROMIUM, AND VANADIUM. THE OXYGEN CONTENT EXCEEDS 0.15%, AND THE CARBON CONTENT IS RELATED TO THE OXYGEN CONTENT AND TO THE THEORETICAL CARBON CONTENT WHICH WOULD STOICHIOMETRICALLY BE NECESSARY FOR FORMING CARBIDES ITH ALLOYING ELEMENTS. POROUS BILLETS ARE PRODUCED BY HEATING THE POWDER IN A VACUUM TO 900-1250*C. TO REDUCE THE OXYGEN CONTENT TO 0.05%.

United States Patent Oflice 3,704,115 Patented Nov. 28, 1972 3,704,115 HIGH ALLOY STEEL POWDERS AND THEIR CONSOLIDATION INTO HOMOGENEOUS TOOL STEEL Erik Goran Wastenson, Viken, and Georg Heinrich Arthur Gerhard Bockstiegel, Hoganas, Sweden, assignors to Hoganas AB, Hoganas, Sweden No Drawing. Filed Feb. 23, 1971, Ser. No. 118,186 Claims priority, application Sweden, Aug. 28, 1970, 11,689/70 Int. Cl. B22f 9/00 Cl. 75-.5 BC 4 Claims ABSTRACT OF THE DISCLDSURE The high alloy steel powder contains one or more elements with a strong affinity to oxygen, such as titanium chromium, and vanadium.

The oxygen content exceeds 0.15%, and the carbon content is related to the oxygen content and to the theoretical carbon content which would stoichiometrically be necessary for forming carbides with the alloying elements. Porous billets are produced by heating the powder in a vacuum to 900-1250 C. to reduce the oxygen content to 0.05%.

The present invention relates to powder metallurgical processes for obtaining high-alloyed homogeneous steels, e.g., high speed steels, with good temper resistance and high red hardness, as well as with a density close to about 100% of the theoretical density. More particularly the invention concerns steel powders with special properties with regard to the carbon and oxygen content, as well as a method of producing from these materials porous billets from which high-alloy steels can be manufactured by means of known methods.

High alloy steel with high carbon content, such as high speed steel and tool steel, is characterized metallographically by a two-phase structure, consisting of carbides embedded in a steel matrix. The distribution and size of these carbides strongly affect such important properties of the material as hardenability, grindability, and mechanical strength. These properties are improved if the steel contains very fine and uniformly distributed carbide particles in a fine-grained matrix.

An advantageously uniform carbide distribution cannot be achieved in steel processing by starting from an ingot. During solidification carbides of eutectoid type precipitate and form a network in the ingot. The coarseness of this carbide network depends on the cooling rate of the ingot and so indirectly on the size of the mould. It is certainly possible to break up a large proportion of the carbide network by subsequent forging or rolling, assuming that the applied cross-sectional area reduction is suflicient. Yet the possible dimensions of the semi-finished product after completed hot working are restricted by an upper limit depending on the size of the ingot and a lower limit depending on the area reduction, and it is not possible to obtain by this method a completely uniform distribution of carbides in the finished material.

If, on the other hand, a steel material is produced in the form of powder by atomizing a steel melt, every powder particle solidifies almost instantaneously, whereby an extremely fine-grained structure with the desired random and uniform distribution of carbides is achieved. Methods of hot compression of capsules containing such a powder to densities close to about 100% of the theoretical are well known. A prerequisite for obtaining high density values, besides high pressure and high temperature, is that the surface of an individual powder particle has a composition that does not substantially depart from that at its centre, i.e. that the surface of the powder is substantially free from metal oxides. The limiting value for the maximum admissible oxygen content lies below 0.05% will henceforth refer to weight-%). Since the aforesaid types of material contain such alloying elements as e.g. chromium and vanadium with a high aflinity to oxygen, and since the surface exposed to the atmosphere is very large, it is very difiicult to avoid oxidation of the powder particles during the atomization. By atomizing and cooling the powder in an inert gas, such as nitrogen or argon, the oxygen content can be greatly reduced as compared to atomizing in water or steam. The maximum value of 0.05% referred to above is, however, very difiicult to maintain even when atomizing in an inert gas. Even though it is known and technically feasible to produce high-alloyed material containing alloying elements highly susceptible to oxidation with oxygen contents below 0.05% by atomizing and cooling the powder in argon gas (see International Journal of Powder Metallurgy, 4 (3), 1969, pp. 7-17), this technique involves very high working costs owing to the high consumption of the inert gas and high capital outlay.

Methods of hot compaction of capsules containing an alloy-steel powder to densities close to of the theoretical density are well known, for instance from US. patent specifications 2,725,288 and 2,235,958, British patent specification 842,226, Canadian patent specification 616,393, and Swedish display-specification (utlaggningskrift) 315,085. The conventional methods involve the following fundamental processing steps:

(1) Filling the capsule with steel powder;

(2) Pre-compaction of the powder capsule at room temperature;

(3) Pre-heating the powder capsule;

(4) Hot compression at 900-1200 C. by so-called isostatic compression, by extrusion, or by compacting the capsule in a water-cooled die.

Pre-compaction serves to improve the contact between the powder particles, which facilitates heating up to the compaction temperature.

It is also known that by coating or mixing the powder with a carbon-containing material, e.g. lampblack, a high proportion of these surface oxides can be reduced if the capsules are evacuated during pre-heating to the compaction temperature (see U.S. patent specification 3,341,325). For this purpose the powder capsule is connected to a vacuum pump. The difficulty of this method lies in getting all of the carbon to react with the oxides, so that no elementary carbon is left after degassing.

The hitherto known methods of producing high alloy steel by hot compaction of the powder material enclosed in a capsule have had this in common that the composition of the pre-alloyed steel powder has been wholly identical with the desired analysis of the finished material.

The present invention provides a powder material for the production of high alloy steel with good temper resistance and high hot hardness, the powder material being a high alloy steel powder containing alloying elements including at least one element with a strong afiinity to oxygen, in which:

tot

C is the carbon content of the powder in weight percent,

Csmch is the carbon content which would stoichiometrically be necessary for forming carbides with the alloying elements, in weight percent,

O is the total oxygen content of the powder in weight percent, and

k and k are constants which can assume values between 0.7 and 1.0.

The disadvantages and difliculties of the previously known methods of powder-metallurgical production of tool steel are overcome or eliminated at least in their most important aspects by the present invention. The process starts with a powdered steel material, which may contain the conventional alloying substances, such as molybdenum, tungsten, and cobalt, as well as such oxidationsensitive alloying elements as titanium, chromium, and vanadium. The powder material is a high alloy steel powder which preferably contains 10% or more of a carbide-forming addition. The carbide-forming elements may be chosen within the following proportional ranges: Cr 30%, M0 0-20%, W 020%, and V 020%; other carbide-forming elements such as Ti, Ta, Nb, Zr, and Hf, each in the range 0 10%, may also be used. In addition to the carbide-forming addition Co 030% and Al 0-l0% may be included. The total content of alloying elements should not exceed 60%.

The characteristic of the new powder material used here is that its carbon content is higher than that which is necessary to balance the constituent alloying elements in the proportion corresponding to the stoichiometry of the formed carbines. The carbon content is further determined by the total oxygen content of the powder material in accordance with the equation given above. The carbon content will usually be from 0.6 to 5%. The powder particles preferably should, in so far as possible, be of irregular shape and not spherical.

As will be described below, the invention also includes a method for further treatment of the powder material with the intention, of finally obtaining high alloy steel having a density that is equal or substantially equal to the theoretical density.

The optimal properties of a tool steel are achieved if the carbon content bears a certain relation to the quantity of carbide-forming elements, e.g. chromium, tungsten, molybdenum, and vanadium, contained in the steel.

The ratio of the carbon content to the contents of the alloying elements is determined by the stoichiometric composition of the carbides which are formed when the steel is tempered, for example Cr C W C, Mo C and V C The required quantity of carbon (C for the stoichiometric equilibrium with some alloying elements will be found in Table 1.

TABLE 1 Cptoieh per 1% of alloying element, percent Alloying element Carbide Tungsten Molybdenu Chr0mium The following values (see Table 2) of the ratio Carbon Content: Csmch are obtained for some standardized high speed steels:

As will be seen from Table 2, no high speed steel has a ratio of greater than unity. The reason for this is that the proportion of retained austenite after hardening increases very rapidly with rising carbon content. As an example it may be mentioned that in the case of the high speed steel AISI M2 an increase in carbon content from 0.85 to 1.1% (ratio carbon content: C =l) entails a rise in the amount of retained austenite from 25 to 50-55% (see Trans. of the ASM, 57 (1964), pp. 925- 948). This proportion of retained austenite can be eliminated by multiple tempering and a high secondary hardness can be attained. With a carbon content of 1.3% (carbon content: C =l.l) there is 70% of retained austenite. This high content of retained austenite makes impossible any successful heat treatment of the steel. Thus the carbon content of 1.1% represents the upper practical limit in a high speed steel of the type AISI M2.

According to the present invention a powder is produced with a total carbon content, C which is considerably above that required for forming the alloy carbides, i.e. Ct t/C t 1 1-1.

The carbon content of the powder material is determined by the composition of the steel envisaged as the end product and by the total oxygen content, O of the powder material after atomization. Experimental data based on a large number of tests has been condensed in the following equation which relates the carbon content to the content of alloyiing elements and the total oxygen content of the material:

where k and k can assume values between 0.7 and 1.0. This implies that the total oxygen content, O of the material must be so high that the ratio C /C exceeds 1.1. It has been proven in practice that the oxygen content must not fall below 0.15%.

One suitable composition of the powder material is Cr 3.5 to 4.5%, M0 3.5 to 5%, W 5 to 7%, V 2 to 4%, Co 5 to 10%, carbon 1.15 to 2.0%, and balance Fe and impurities including oxygen.

Powder is produced by atomizing with water or steam whereby every powder particle is subjected to almost instantaneous cooling. This procedure yields a very finegrained and uniform carbide structure in every powder particle. Owing to the high carbon content, the M temperature, the temperature at which martensite begins to form, will be lowered to below C. As a consequence, in spite of the rapid cooling, the austenite becomes so stabilized that the amount of retained austenite exceeds 70%. This high austenite content leads to a great reduction in hardness for every individual powder particle after atomizing as compared with conventional powders produced with the usual carbon contents. Owing to this decrease in hardness the compressibility of the powder is improved.

In the atomization process the surface of the particles becomes oxidized. This causes the oxygen content of the powder to exceed 0.15 The combination of high carbon content with oxygen content gives the powder unique properties, which makes it very suitable as raw material for powder metallurgical production of tool steel. By heat treating the powder in a vacuum the carbon dissolved in the steel is caused to reduce the surface oxides, provided that a sufficiently low pressure, i.e. less than 10- torr, is maintained. Since the carbon is uniformly distributed in each individual powder particle, the transport path of the carbon or carbon-bearing gases to the surface oxides is the shortest conceivable in practice. Owing to this, very low oxygen contents can be obtained by reduction of the powder. This is explained in the examples given below.

EXAMPLE 1 With a water-atomized high-speed steel powder AISI M2 (analysis: C 0.78, Cr 4.45, Mo 5.10, V 1.84, W 5.28), ---40 mesh, having a total oxygen content of 0.28%,

graphite was admixed in amounts between 0.1 and 0.3%. The various powder mixtures were then put into cylindrical capsules 60 mm. diameter x 100 mm., and compressed in these at room temperature to a density 62% of the theoretical density. Thereafter the powder capsules were heated up to 150C, the interior of the capsule being steadily evacuated at the same time. This temperature was maintained for 4 hours. The pressure in the capsules at the end of the treatment was about torr. The following results were obtained upon the completion of the heat treatment and isostatic hot compacting at 1150 C.:

Compacted powder Density, percent of the Carbon, Total oxytheoretical Graphite admixture, percent percent gen, percent -In the metallographic investigation of the compacted and heat treated specimens, the presence of residual graphite particles and unreduced oxides in all the above specimens was established.

After being removed from the capsule, the material with the highest density after hot compacting, i.e. that with a 0.2% graphite admixture, was subjected to hot forging at 1185 C., whereby the volume of the specimen was reduced by 36%. In a metallographic investigation of the forged billet it was clearly seen that the porosities arising from the residual graphite and unreduced oxide particles persisted even after forging. Thus it was not possible to obtain full density after the second processing step.

EXAMPLE 2 A water-atomized high-speed steel powder AISI M2 (analysis: Cr 4.47, Mo 5.09, V 1.72, W 5.92), mesh, having a carbon content of 1.35% (C /C =1.24) was enclosed in the same way as in Example 1 in a capsule 60 mm. diameter x 100 mm. and compacted to a density of 68% of the theoretical density. Thereafter the powder capsule was heated up to 1150 C. with simultaneous evacuation. This temperature was maintained for 1 hour. At the end of this time the pressure in the capsule was 10- torr. Upon the completion of the heat treatment the capsule was closed and then isostatically hot compacted under a pressure of 1.5 kbar. The temperature was 1150 C. as in the earlier example. The following results were obtained:

Density, percent of theoretical.

As in Example 1 this billet was subjected to a second hot working by forging. The forging temperature was 1160 C. and the degree of reduction 34%. No residual porosity at all was found in subsequent metallographic investigation and the material was completely dense.

Example 1 shows that a decrease in the oxygen content of the atomized high alloy steel has been made possible by reduction with admixed carbon under vacuum and at an elevated temperature. The decrease, however, has been altogether insufiicient and the specimen obtained after the heat treatment could not be worked into an acceptable tool steel.

In Example 2 carbon has been added to the steel melt before atomization in such an amount as to obtain the stabilization of the austenite during the rapid cooling of the powder particles. This results in a somewhat enhanced compressibility and a higher density after the compacting than in Example 1. The reduction of the oxides in the vacuum heat treatment of the pre-compacted powder body is further greatly increased by having the excess carbon required for the reactions in solution in every powder particle, unlike Example 1 where the carbon has been introduced by admixture.

Atomizing with a gas results in powder particles of spherical shape. A relatively high apparent density is certainly obtained with such a powder, but owing to the combination of the great hardness with the spherical particle shape, this powder cannot be pressed into compacts of such mechanical strength, so-called green strength, that the pressed materials can be handled without disintegrating. It is, therefore, necessary to encapsulate the powder before the vacuum treatment. If water is used instead as the atomizing medium, irregular powder particles are obtained. These can be pressed even in the cold state into compacts of suflicient green strength to enable the pressed material to be handled without falling apart.

By using a powder according to the present invention the green strength is further increased, because these powders can, owing to their high carbon content, and hence lower hardness, be compressed to a higher density.

If hot isostatic compaction is employed for the final consolidation, encapsulation is necessary to make compaction possible. When a powder according to the present invention is used it may be advantageous not to effect the encapsulation of the material till after the pre-compacting and reduction treatment has been carried out. Thus it has proved that the evacuation of the pre-compacted powder capsule can be more effective if it is not enclosed in a sheet steel capsule. This is due to the lowest possible resistance to the flow of the escaping reaction products being ensured if the gases can leave the whole of the surface of the compact rather than a small portion thereof, which is the case if the encapsulation of the powder takes place before the evacuation treatment as previously described. This is shown in the following example:

EXAMPLE 3 The same high speed steel powder with high carbon content as in Example 2 was pressed by isostatic compaction (using a rubber mould which was subsequently removed) under a pressure of 3 kbar into a compact having the dimension 75 x mm. The density of the compact was 5.33 g./cm. i.e. 64% of the theoretical. Subsequently the compact was heat treated in vacuum at a temperature of 1150 C. The pressure in the furnace was 10* torr throughout the treatment period at 1150 C. Upon the completion of the heat treatment the furnace was filled with argon and cooled to room temperature. The powder body was then put into a sheet steel capsule and hot isostatically compacted with 1.5 kbar at 1150 C. The capsule containing the powder body was evacuated before being heated up. The density after hot compaction was 99100% of the theoretical and the oxygen content was 0.003%.

Thus the porous body which is obtained after the cold compacting and reduction treatment forms a suitable material for further consolidation by hot isostatic compacting, by extrusion, or by pressing in a die. In order to make isostatic compaction possible, and to protect the material against oxidation While it is being heated up to a suitable processing temperature, it must be enclosed in a pressuretight capsule made of sheet steel. Unless the capsule is evacuated during the encapsulation itself, it must be provided with an evacuation tube in order to make possible the evacuation of entrapped air before or during the heating up to a temperature suitable for hot compacting. Through this tube an inert gas, e.g. argon, can also be introduced into the capsule, to protect it against oxidation during prolonged storage.

In some cases it has proved advantageous to put the hot body or billet in a pressure-tight container or capsule as soon as a sufiiciently low oxygen content has been reached during the reduction treatment, the pressure thus being maintained or reduced. This ensures the advantage that the encapsulated material, while still hot, can be transferred into a compacting apparatus, which saves an additional reheating operation.

When a powder according to the invention is cold compacted, powder compacts can be formed whose geometrical shape corresponds to that of the final product, as in the case of the conventional powder-metallurgical methods. If this shape is complicated the powder compact is difficult to encapsulate, so that isostatic hot compaction is unsuitable. It has proved, however, that the theoretical density can be reached by forging a powder material of complicated shape, starting from a powder according to the present invention and produced by cold compaction and a reducing treatment as above, provided that the body is shielded against oxidation either by carrying out the forging in an inert atmosphere, or vacuum, or by protecting the material by a chemical protective coating. This is elucidated in the following example:

EXAMPLE 4 An annular body 25 mm. outside diameter and 9 mm. thick was produced by cold pressing in a compacting die. After reduction treatment in a vacuum furnace at 1150 C., as in Example 3, and subsequent cooling down to room temperature, the body was coated with a chemical protective layer, the main constituent of which was water glass. Thereafter the body was induction-heated to 1150 C. in an argon atmosphere. After heating, the body was quickly transferred into a pre-heated forging die, the forging then being carried out, the time of transfer from the furnace to the die being less than 3 seconds. The density after forging exceeded 99% of the theoretical density.

We claim:

1. A powder material for the production of high alloy steel with good temper resistance and high hot hardness by hot compaction, the powder material being a high alloy steel powder consisting, in weight percent, of Co -30, Al 0-10, carbon 0.6-5, and at least 10% of a carbide-forming addition consisting of at least one carbideforming element selected from the group consisting of Cr 0-30, Mo 0-20, W 0-20, V 0-20, Ti 0-10, Ta 0-10, Nb 0-10 Zr 0-10, and Hf 0-10; the total content of alloying elements not exceeding 60% by weight; the balance being Fe and impurities including oxygen; and in which:

tot 1 atoicb 1 and tot stolch+ tot where C is the carbon content of the powder in weight percent, Csmch is the carbon content which would stoichiometrically be necessary for forming carbides with the alloying elements, in weight percent,

O is the total oxygen content of the powder in weight percent, and

k and k are constants which can assume values between 0.7 and 1.0.

2. A powder material for the production of high alloy steel with good temper resistance and high hot hardness by hot compaction, the powder material being a high alloy steel powder consisting, in weight percent, of Co 11-30, carbon 0.6-5, and at least 10% of a carbide-forming addition consisting of at least one carbide-forming element selected from the group consisting of Cr 0-30, Mo 0-20, W 0-20, and V O-20; the total content of alloying elements not exceeding by weight; the balance being Fe and impurities including oxygen; and in which:

tot 1.1 stoich and tot= stoich+ tot where C is the carbon content of the powder in weight percent,

0 is the carbon content which would stoichiometrically be necessary for forming carbides with the alloying elements, in weight percent,

O is the total oxygen content of the powder in Weight percent, and

k and k are constants which can assume values between 0.7 and 1.0.

3. A powder material as claimed in claim 1, wherein the shape of the particles is substantially irregular.

4. A powder material as claimed in claim 1, consisting of Cr 3.5 to 4.5%, M0 3.5 to 5%, W 5 to 7%, V 2 to 4%, Co 5 to 10%, carbon 1.15 to 2.0%, and balance Fe and impurities including oxygen.

References Cited UNITED STATES PATENTS I WA, 

